Adhesives or binders are a decisive constituent in the industrial production of a multitude of products, for instance in the production of wooden composite boards. Hereby, wood chipping products are coated with a binder and compressed to molded articles such as boards by applying pressure and temperature. The type and amount of the adhesive used is thereby substantially influenced by the size and quality of the wood fibers and/or wood chips used.
For example, in the production of MDF or HDF boards wood fibers are typically used in combination with formaldehyde resins, as for instance melamine formaldehyde resins or urea formaldehyde resins, whereas wood chips (strands) for producing OSB boards (oriented strand board) are used in combination with polyurethane binders, for instance on PMDI basis.
The strand-shaped waste obtained originally in the veneer and plywood industry were processed at first in small amounts to OSB boards, which were used for different applications. Today, they are mainly used in the wood and pre-fabricated house construction since OSB boards are light-weighted and can still fulfill the static requirements defined for building panels. Thus, OSB boards are used as structural boards and as wall or roof panels or also in the floor field.
The production of OSB boards takes place in a multistage process, wherein at first the chips or strands are cut out of debarked round wood, in particular soft woods, in longitudinal direction by rotating knifes. In the subsequent drying process the natural moisture of the strands is reduced at high temperatures. The moisture content of the strands can vary depending on the adhesive used. Thus, depending on the adhesive, a wetting of more moist strands or of dry strands can be more favorable. Thereby, a variation of the moisture of the strands is only possible to a small degree such that especially the adhesive has to be adapted to the present moisture. Furthermore, as little as possible moisture should be present in the strands during the pressing step in order to reduce the vapor pressure generated during the pressing step as much as possible since this could otherwise promote a bursting of the raw board.
Following the drying of the strands they are fed into an apparatus, in which the glue or adhesive is applied finely dispersed on the chips. Predominantly, PMDI (Polymeric Diphenyl Methane Diisocyanate) or MUPF glues (Melamine-Urea-Phenol-Formaldehyde) are used for gluing. The glues are applied preferably in singular layers. The glues can also be used in the OSB board in a mixed manner. These glues are used since the OSB boards are often used for construction applications as mentioned above. There moisture or wet resistant glues have to be used.
After providing the glue the glued strands are scattered in scattering apparatus alternating lengthwise and crosswise to the production direction such that the strands are arranged crosswise in at least three layers (lower layer-middle layer-upper layer, wherein the scattering direction of lower and upper layer are the same, deviate however from the scattering direction of the middle layer).
Following the scattering of the strands a continuous pressing of the same is carried out at high pressure and high temperature at for instance 200 to 250° C.
As studies have shown a penetration or diffusion of the glue into the wood is to be observed when gluing strands from soft wood (for instance with PMDI). One reason for this is that in particular the hydrophobic PMDI glue is absorbed by the wood surface, which contains depending on the time of the year more or less fatty acids. Therefore, only a part of the glue can be used for gluing the strands on the surface.
In particular, when producing OSB boards additional expenses for glue or adhesive amounts are necessary in order to compensate for the penetration or absorption of the applied glue into the wood chips or strands. This causes significant additional costs.